The field of fire protection is of extreme importance to life and equipment. Firefighting system design presents many challenges to the designer including a need to optimize the effectiveness of the fire protection equipment while at the same time minimizing the cost of installation. Improving fire protection calls for rapid delivery of fire suppression fluid such as water, foam, or gas, to overcome fires. Fires grow rapidly and therefore methods to achieve early fire suppression can significantly reduce overall installation costs by minimizing the scale of the system needed to suppress the fire. Rapid delivery design requirements can drive a need for large pipe and valve diameters, and/or a need for high operating pressures to accelerate fluid delivery. Clearly, high reliability is of-course of high importance.
The most common sprinkler systems utilize water as fire suppressant as in many locations water supply is available at sufficiently high pressure and volume, often from public distribution mains. Thus the present specifications will utilize the term ‘water’ as equivalent shorthand to any relevant primary fire suppressant fluid, by way of non-limiting example.
Sprinkler systems are standardized nowadays to deliver the fire suppression fluid to the needed sites. Sprinkler systems comprise a fluid distribution system, having pipes that communicates fluid to a plurality of individual sprinklers. The sprinklers act as heat sensitive elements as well as fire suppressant fluid outlets. In certain sprinkler systems commonly known as wet systems, fluid, such as water, is present at the sprinkler. As the heat caused by the fire is the reason of the activation of the sprinkler, the systems delivers fire suppressant from at least one sprinkler near the fire relatively rapidly, namely as soon as the sprinkler acting as a heat sensing elements is activated.
Certain implementations of sprinkler systems, such as outdoor systems in cold geographical locations or cold storage facilities by way of example, present freezing risk if the water are kept in the distribution system. To resolve this problem many sprinkler systems are dry —water is introduced to the system only in case of detection of fire. Controlling the introduction of the water is done by a control valve.
Both wet and dry sprinkler systems utilize other valves as well—a shutoff valve is utilized to control introduction of water to the system as a whole for purposes of construction and maintenance, for handling leaks, and of-course after a fire system is activated and the fire is extinguished, to allow stopping of water flow, and bringing the system back to operational state.
Wet sprinkler systems commonly utilize a check valve to maintain water in the distribution system which may otherwise vary due to pressure and temperature fluctuations. Most common check valves in such systems utilize a movable sealing element colloquially known as a clapper, sealing against a seat. The seat and sealing member interface defines a sealing port of the valve. The pressure differential on both sides of the clapper is relatively small, and check valves commonly have similar areas on both sides of the clapper exposed to the fluid. Check valves often use gravity, springs or and the like to urge the clapper to a closed state and allow flow only in one direction.
Check valves are also used in different portions of the distribution system to protect specific zones such as differing floors in multi-floor buildings, differing zones of warehouses, and the like.
Clappers are also commonly utilized in “dry pipe” system control valves. Dry pipe sprinkler systems control introduction of water by having a fluid such as compressed gas in the distribution system, however the gas pressure is far lower than the water pressure. A control valve is equipped with a clapper and is constructed such that a larger area of the clapper is exposed to gas pressure than the area exposed to water pressure. This provides a mechanical advantage which maintain the control valve closed. Subsequent to a sprinkler activation, gas is released from the distribution system, the gas pressure drops and the water pressure is sufficient to overcome the gas pressure and water are introduced to fight the distribution system.
In a fire fighting system the flow resistance presented by any component may be critical to the system performance, and this extends to check valves—while flow resistance of the check valve is important in many applications, it is a critical consideration in firefighting systems.
Sensing the existence of fire, and promptly alerting personnel thereof is highly important to provide personal security and to minimize property damage. To that end flow sensors are often utilized to provide an indication of flow resulting from activation of at least one sprinkler. In certain cases flow detection may be utilized for additional purposes, such as activating a pump, and the like. Flow detection may take place relevant to primary fluid flow, such as the water, and/or to a secondary fluid such as the compressed gas. Detecting flow of primary, and/or in certain cases detecting flow of secondary fluid flow, activates alarms in various firefighting system arrangements. If a flow detector acts as a switch to activate an alarm or otherwise report system activation when even a single sprinkler trips, appropriate personnel may take action to evacuate an area and take other actions to mitigate the fire effects. While these specifications use, by way of convenient example water based wet system it is noted that all firefighting system types, including various dry systems known as deluge, dry pilot, wet pilot, and the like, may benefit from flow detection, for activating alarm, for testing, for activating a booster pump, and the like.
However, experience shows that flow may sometimes exist for a short period, due to transient effects of pressure and/or temperature, without true occurrence of a fire. In order to minimize false alarms resulting from such transient variations flow switches in firefighting systems often utilize timing devices to delay the onset of the alarm for a predetermined period.
Flow sensors are known in the art, and are primarily embodied in a paddle or a vane. By way of example U.S. Pat. No. 2,873,606 to Ekstrom, presents one flow detector however it is often considered too restrictive for firefighting purposes. U.S. Pat. No. 6,239,446 to Cholin is but one example disclosing a flow detector utilizing a paddle disposed in the firefighting system riser to detect fluid flow. One problem with paddle or vane based systems is that they require placing a flow disturbance in the main fluid line. Furthermore, the detector must be sufficiently sensitive to detect relatively small flow, such as may be caused by a single sprinkler in a riser having a cross section area which is a large multiple of the cross section surface of the orifice of the sprinkler. This requires large paddle surface relative to the riser size, as well as minimal resistance to the vane movement which is transmitted outside the riser. Producing such minimal resistance makes sealing the point where the paddle leaves the riser a weakness point. Furthermore, the paddle solution mandates specific size paddle for each riser size, and commonly further requires drilling a hole into the riser, all of which increases stock requirements, complexity and cost. Additionally, in order to increase the sensitivity of the flow sensor to low flow, the paddle is oftentimes made of light, low strength material. Such paddle or portions thereof may be torn from the flow sensor during the full flow condition caused by large fire, and drift to a portion of the distribution system where they may block water flow at the most critical time.
Differential fluid detectors are also known. Such flow detectors are based on measuring fluid pressure on two sides of a restriction placed in fluid path. By way of example, U.S. Pat. No. 4,221,134 provides for fluid flow measurement utilizing such a restriction. However again, such devices are considered inappropriate for firefighting systems, as they restrict flow therethrough. It is a common and clearly understood requirement for a firefighting system to minimize fluid flow restrictions as a large fire requires as much of the firefighting fluid available in the shortest possible period.
It is seen that the design of a fire suppression system such as sprinkler systems benefits from reliable flow detection of relatively small amounts of fluid relative to the total system capacity or even to the downstream portion of the system from the fluid flow detector. There is therefore an ongoing need for a simple, reliable, and sufficiently sensitive fluid flow detection methods and devices in such firefighting systems, while minimizing flow restriction during system activation.